1. Field of the Invention
The present invention relates to a mobile antenna mounted on a mobile such as an automobile and, in particular, to a mobile antenna having a small size and a small height in a mounted state and suitable for diversity reception.
2. Description of the Related Art
With the recent rapid progress of electronic communication technique, communication apparatuses having a higher function and a smaller size have been developed and utilized for various kinds of mobile communication apparatuses. Especially, mobile telephones have already become widespread due to their convenience.
In such a mobile communication apparatus, an antenna which is mounted on a mobile has a very important role. That is, in a mobile communication apparatus such as a mobile telephone, radio waves must be transmitted and received between a mobile which changes the position thereof every moment and a fixed base station, and communication is impossible without sufficient transmission and reception on the side of the antenna mounted on the mobile.
A rod-like antenna such as a dipole antenna has conventionally been used widely as a mobile antenna. This is because a dipole antenna is considered to be suitable for transmitting and receiving a vertically polarized wave which is used for a mobile communication apparatus such as a mobile telephone.
A dipole antenna, however, must have a length of about half the wavelength of the radio wave which is used for communication, for example, about 16.7 cm in the case of a radio wave of 900 MHz which is used for a mobile telephone. If such a long antenna protruding from a vehicle body is mounted on an automobile, it may be broken and there is also a problem in aesthetic appearance.
To solve these problems, inverted F antenna such as that shown in FIG. 12, loop antenna such as that shown in FIG. 13, and table antenna such as that shown in FIG. 14, etc. have conventionally been proposed as an antenna having a small height in a mounted state.
The inverted F antenna shown in FIG. 12 has a structure in which one end of a radiating element 12 disposed on a ground plane 10 is bent so as to be connected to the ground plate 10.
The length L.sub.2 of the radiating element 12 is about 1/4 of the wavelength .lambda.g of the propagated wave. The inner conductor 14a of a coaxial feeder 14 is connected to a point which is d.sub.l distant from the bent portion of the radiating element 12 for the purpose of the impedance matching between the coaxial feeder 14 consisting of the inner conductor 14a and an outer conductor 14b and the radiating element 12. By adjusting the distance d.sub.l, it is possible to adjust the input impedance at the feeding point of the antenna in conformity with the impedance (usually about 50 .OMEGA.) of the coaxial feeder 14.
In this way, it is possible to transmit and receive a predetermined radio wave from and by the radiating element 12 by the current supplied from the coaxial feeder 14.
The loop antenna shown in FIG. 13 has a structure in which the coaxial feeder 14 is protruded from the ground plane 10 with the inner conductor 14a thereof formed into an arcuate loop having a length of Lp with the other end of the loop in contact with the ground plane 10. The height of the loop 12 from the ground plane 10 is set at Hp.
According to this structure, the antenna resonates at the frequency at which the length Lp of the loop 12 is about 1/2 of the wavelength of the radio wave transmitted and received. Transmission and reception are therefore possible at this frequency.
FIG. 14 shows a table antenna. This table antenna has a structure in which a circular radiating element (table) 12 having a diameter of Dt is supported by four conductor posts 16 having a height of ht and disposed on the ground plane 10, and the inner conductor 14a of the coaxial feeder 14 is connected to the central part of the radiating element 12.
In this table antenna, feeding is conducted through the inner conductor 14a of the coaxial feeder 14 connected to the central part of the table 12 which is horizontally placed.
Current I.sub.1 thus radially flows from the feeding point to the four posts 16 and the antenna resonates at a frequency at which the wavelength of the radio wave is equal to about twice the path length of the current.
In this table antenna, a relative band width is as broad as about 10%, and in this respect it is considered to be suitable for a mobile communication antenna.
Mobile communication is frequently influenced by the reflection and scattering of the radio wave due to buildings and the like while the mobile is travelling in an urban district. A mobile communication apparatus mainly conducts communication in an environment of multipath propagation caused by the scattering or reflection of the radio wave, so that it is impossible to avoid the deterioration of the communication quality due to the generation of fading.
One of the methods for lightening the influence of the fading phenomenon is diversity reception. Diversity reception is a method of improving the communication quality by arranging a plurality of (usually two) antennas with a predetermined space therebetween and automatically switching the current antenna over to the antenna which has received a signal at a higher level or compounding the signals received by the respective antennas.
In the antennas used for diversity reception, it is important that the correlation between the received signals is small. For this purpose, it is necessary to arrange the antennas such that the mutual coupling between the two antennas is as small as possible.
In this case, it is possible to make the coupling level sufficiently low by broadening the space between the two antennas. It is, however, impossible to arrange the two antennas with a large space therebetween due to the limited size of a mobile. As a countermeasure, two dipole antennas with one placed on top of the other on a vertical line is conventionally used for achieving the diversity reception in a mobile. This structure enables antennas to be mounted on a small mobile.
As described above, antennas having a small size and a small height in a mounted state are conventionally proposed. These antennas, however, the following problems.
In an inverted F antenna, the direction in which the radiation of the radio wave from the radiating element 12 is the maximum has such a high elevation that sufficient transmission and reception from and by the base station on the ground is impossible. In addition, since the direction of flow of the current in the radiating element 12 is limitative, it is impossible to obtain an antenna which has an omni-directional pattern in a horizontal plane. The transmission and reception sensitivity therefore depends upon the direction in which the base station is located. Furthermore, the relative band width is disadvantageously narrow.
A loop antenna, which has a very simple structure, is considered to be suitable as a mobile antenna. However, if the loop antenna has a small height in a mounted state, namely, if the height Hp is smaller than the width Wp, the capacitance between the radiating element line and the ground plane becomes large, the impedance becomes capacitive and the radiation resistance becomes small. (In the extreme case in which Hp is 0, the radiation resistance becomes 0). That is, if a loop antenna has a small height in a mounted structure, it is difficult to obtain matching between the radiating element 12 and the coaxial feeder 14 and the band width becomes disadvantageously narrow.
The resonance frequency of a table antenna is a frequency at which the current path Lt has a length equivalent to about 1/2 wavelength, as described above. EQU Lt=2.times.ht+Dt
wherein ht represents the height of a table and Dt a diameter of the table.
Therefore, if the height ht is reduced, it is necessary that the diameter Dt of the table must be about 1/2 wavelength (for example, if the radio wave has a frequency of 900 MHz, the diameter is about 16.7 cm), and the antenna cannot be said to have a small size.
Especially, if two antennas of this type are used for diversity reception, the size of the antenna system becomes considerably large and the coupling level of the two antennas becomes very high.
In order to achieve diversity reception in a mobile antenna, it is necessary to make the coupling level as low as possible. Mere arrangement of the conventional small-sized antennas described above, however, disadvantageously increases the coupling level. In the case of vertically placing one antenna on top of another, the height of the antenna system becomes very large.